Method and circuit for measuring a current

ABSTRACT

A method and circuit arrangement for measuring a current are described, which can be used in electrical engineering to precisely trigger electronic tripping devices in circuit breakers. The current is measured according to magnitude and phase in such a way that only the fundamental wave of the current between precisely determined current zero crossings is sampled at a low sampling rate, and the root mean square is calculated on the basis of the measured phase angle and amplitude. The exact phase angle is measured using a conventional PLL circuit with a downstream zero switch.

FIELD OF THE INVENTION

The present invention relates to a method and a circuit arrangement formeasuring a current according to magnitude and phase angle and can beused, for example, to precisely trigger electronic tripping devices incircuit breakers.

BACKGROUND INFORMATION

Precise compliance with specified tripping characteristics and displayof current values is now required for electronic tripping devices incircuit breakers. One conventional method of increasing currentmeasurement precision is to use Rogowski coils with a downstreamintegrator and normalizing amplifier.

A further conventional method of enhancing precision is to increase thesampling rate of the analog signal and improve the software analysis ofthe sampled values.

A method for sampling measured quantities and further processing thesampled measured quantities to generate a tripping signal as a functionof the magnitude and duration of an overcurrent is described in GermanPatent No. 35 22 739. According to this method, the sampled measuredvalues are converted to digital values that are weighted and added upaccording to a specified function. The tripping signal is formed when acumulative value lies above a specified reference value.

Increasing the sampling rate is a common conventional method. However,this method requires powerful processors that are relatively expensiveand consume a great deal of power. The price and power consumption thuslimit the ability to increase the sampling rate for economic andtechnical reasons.

Improving precision through software analysis is based on the fact thata sinusoidal value—the fundamental wave in this is uniquely determinedby a pair of values, namely the magnitude and angle. Thus, one sampledvalue per period would be entirely sufficient to determine the currentwith an adequate degree of precision in currents without harmonics.

However, these idealized conditions do not exist in practice. Instead,the practical reality is that harmonics do occur in operating currents,and there is the difficulty of precisely determining the phase angle inharmonic currents. For these reasons, current measurement or analysis ofthis type has not been previously used.

Previously, the root mean square was calculated by measuring the largestpossible number of sampled values per period without taking the phaseangle into account. The sampled values were squared, added up over aspecified period of time, and the average thus formed. An electronictripping unit for a circuit-breaker, in which a microcomputer is used todetermine the root mean squate of the conductor current, is described inGerman Patent No. 43 23 356.

SUMMARY

An object of the present invention is to provide a method and a circuitarrangement for measuring a current, using simple means to accuratelymeasure the current and enabling precise, reliably, and reproducibletripping of circuit breakers.

One particular advantage of the present invention is that the trippingprecision of electronic tripping devices in circuit breakers can besignificantly improved at a low cost, using simple means, by measuringthe current according to magnitude and phase so that only thefundamental wave of the current is sampled at a low sampling ratebetween precisely determined current zero crossings, and the root meansquare is calculated on the basis of the measured phase angle and theamplitude. To achieve this, a PLL circuit with a downstream zero switchis provided parallel to the input of the A/D converter and to a portinput of the microprocessor.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE shows an example embodiment of the present invention.

DETAILED DESCRIPTION

The FIGURE shows a simplified single-pole representation, wherereference number 1 designates the main conducting path of a low-voltagecircuit-breaker. A switching contact 2 belonging to main conducting path1 can be latched in the closed switching position in a conventionalmanner by a latching mechanism 3 and can be released for tripping by atripping magnet 4. A Rogowski coil 5 that is connected to an integrator6 and a normalizing amplifier 7 is assigned to main conducting path 1and used to measure the current. The current signals obtained from thiscircuit arrangement are processed in a microprocessor 8 after they havebeen converted by an A/D converter 9 to a form suitable for thispurpose.

It is possible to measure the current according to magnitude and phaseangle because an exact phase angle can be determined using aconventional PLL (phase-locked loop) circuit 10 with a downstream zeroswitch 11, as is conventional, for example, in connection withtelecommunications engineering. The precise phase angle measurement isachieved, even in currents with a large harmonic component, byconnecting PLL circuit 10 parallel to the input of A/D converter 9 andto a port input of microprocessor 8. Controlled oscillator 12 containedin PLL circuit 10 is frequency-limited so that it can respond only tothe fundamental wave of the current. Zero switch 11, which emits,precisely at the current zero, an edge change that is detected via aport input of microprocessor 8, is connected downstream from controlledoscillator 12. Using the processor clock as a time scale and the fixedphase relationship between controlled oscillator 12 and primary current,the software can form a precise value pair, composed of the magnitudeand phase. The software samples, in a controlled manner, the analogcurrent signal, which can be used to very accurately calculate the rootmean square at a low sampling rate.

The present invention is not limited to the example embodimentillustrated here. Instead, it is possible to combine and modify theabove-mentioned means and features to implement further embodimentswithout exceeding the scope of the present invention.

What is claimed is:
 1. A method for measuring a current to generate atripping signal for a circuit breaker, comprising: sampling only afundamental wave of the current between current zero crossings;converting the samples to digital values; determining a magnitude andphase angle as a function of the digital values; determining a root meansquare as a function of the determined magnitude and phase angle; andgenerating the tripping signal for the circuit breaker as a function ofthe root mean square.
 2. The method according to claims 1, wherein thesampling step includes sampling at a low sampling rate.
 3. The methodaccording to claim 1, further comprising: precisely determining thecurrent zero crossings.
 4. The method according to claim 1, wherein thesampling step includes controlling the sampling via software control. 5.A circuit for measuring a current to generate a current to generate atripping signal for a circuit breaker, comprising: an electronictripping device including a switching contact arranged in a mainconducting path, a latching mechanism, a tripping magnet, a Rogowskicoil, a normalized amplifier and a microprocessor with an A/D converter;and a PLL circuit including a downstream zero switch coupled in parallelto an input of the A/D converter and to a port input of themicroprocessor, wherein the microprocessor samples a fundamental wave ofa current and forms precise value pairs each composed of a magnitude anda phase, the microprocessor determining a root mean square as a functionof the value pairs and generating a tripping signal as a function of theroot mean square.
 6. The circuit according to claim 5, wherein the PLLcircuit includes a controlled oscillator that is frequency-limited sothat the controlled oscillator responds only to a fundamental wave ofthe current.
 7. A circuit for measuring a current to generate a currentto generate a tripping signal for a circuit breaker, comprising: anelectronic tripping device including a switching contact arranged in amain conducting path, a latching mechanism, a tripping magnet, aRogowski coil, a normalized amplifier and a microprocessor with an A/Dconverter; and a PLL circuit including a downstream zero switch coupledin parallel to an input of the A/D converter and to a port input of themicroprocessor; wherein the PLL circuit includes a controlled oscillatorthat is frequency-limited so that the controlled oscillator respondsonly to a fundamental wave of the current, and wherein the zero switchemits a signal for an edge change precisely at a zero crossing of thefundamental wave.
 8. A method for measuring a current to generate atripping signal for a circuit breaker, comprising: emitting a signal foran edge change precisely at a zero crossing of a fundamental wave;sampling only the fundamental wave of the current between current zerocrossings of the fundamental wave as a function of the emitted signal;converting the samples to digital values; determining a magnitude andphase angle as a function of the digital values; determining a root meansquare as a function of the determined magnitude and phase angle; andgenerating the tripping signal for the circuit breaker as a function ofthe root mean square.